This invention relates to offshore structures, and more particularly to offshore structures adapted for supporting oil and gas exploration/production operations at sea. Even more particularly, the invention relates to a type of an offshore structure known as a jack-up unit. A typical jack-up unit design uses a floatable hull with three or four supporting legs, which may be circular, square or triangular in cross-section, extending through the hull within leg guides. The legs may be built as truss units using a system of horizontal and diagonal braces. The legs support the hull during offshore operations, and are supported by the hull during transit.
Once the platform is delivered to the desired location, the legs are lowered through openings in the hull to reach the seabed. The legs are secured to the bottom and then the hull is elevated to the operational height. The lowering and raising of the legs is performed by a plurality of jack-up assemblies typically located at the corners of the platform.
In a typical rack and pinion type jacking system, there are a total of nine jacking assemblies, three assemblies per leg of legs having triangular configuration. Each jacking assembly unit comprises four to six pinions, which are housed in a jack frame and supported on bearings. A series of guide plates are installed above and below the jacking mechanism. The guide system consists of upper guide plates, middle guide plates and lower guide plates. Gaps between the guide plates and rack are pre-determined to ensure smooth transition in raising and lowering of the legs.
Conventional assembly of guide plates is shown in FIG. 2. The guide is firmly fixed to the supporting structure. Edge guide plates 1 are located in opposite edges of the rack teeth 2. The edge guide plates 1 allow the rack teeth 2 to support and slide during the jacking process. Initially, when the whole hull unit is resting on the pinions, the differential loads on the pinions cause a vertical moment couple during the jacking up process.
Under environmental loads, the unit tilts and the rack teeth will react against the guide plates 1. This generates a reaction on the guide plates along the chord and indirectly on the horizontal and the diagonal braces 3. The differential loads on the guide plates cause a horizontal moment couple to be developed. As the jacking process continues, the transfer of the loads from a vertical to a horizontal moment couple increases. As a result, the legs between the upper and lower guide plates sustain a large bending moment. Thus the horizontal and diagonal braces between the upper and lower guide plates develop compressive and tensile forces. Since the legs have truss structure, the braces tend to fail under compressive load that is built up due to the horizontal moment couple.
The industry understands that high compressive loads are undesirable as they result in buckling of the braces under severe environmental conditions. For example, when a rig suffers a severe punch through situation or when the spud can at the base of the unit slides into old footings. This guide assembly is inefficient, as the generated high compressive loads located mainly between the upper and lower edge plates. This constitutes a local failure within the system. A premature load buckling of the brace eventually occurs. In the conventional system, since only a few top and bottom plates are reacted, the development of the horizontal moment couple is high. Only a small number of guide plates 1 are sharing the reacted loads.
The capacity of the drilling unit to maintain stability and strength during working conditions is determined by the extent the braces are subjected to the loads through the guide plates 1. Under harsh environmental conditions, the leg structure would deflect and a large bending moment is generated; the large bending moment is reacted against by the guide plates along the rack teeth 2. This reaction generates high compressive loads in the bracing members, which results in failure of the brace by buckling. To overcome this phenomenon, a system of guide plates is installed to significantly reduce the buckling loads exerted on the braces by converting an otherwise compressive load into tensile load.
The present invention contemplates elimination of drawbacks associated with the prior art and provision of an improved system of guide plates in a jacking system.